Ink-jet printing systems commonly utilize either a direct printing or an offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving medium. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving medium is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused into the medium.
In many direct and offset printing systems, the print head moves relative to the final receiving medium or the intermediate transfer surface in two dimensions as the print head jets are fired. Typically, the print head is translated along an X-axis while the final receiving medium/intermediate transfer surface is moved perpendicularly along a Y-axis. In this manner, the print head "scans" over the print medium and forms a dot-matrix image by selectively depositing ink drops at specific locations on the medium.
In a typical offset printing architecture, the print head moves in an X-axis direction that is parallel to the intermediate transfer surface as a drum supporting the surface is rotated. Typically, the print head includes multiple jets configured in a linear array to print a set of scan lines on the intermediate transfer surface with each drum rotation. Precise placement of the scan lines is necessary to meet image resolution requirements and to avoid producing undesired printing artifacts, such as banding and streaking. Accordingly, the Xaxis (head translation) and Y-axis (drum rotation) motions must be carefully coordinated with the firing of the jets to insure proper scan line placement.
Prior ink jet printers have utilized various implementations of a lead screw mechanism to impart X-axis movement to a print head. An exemplary patent that discloses a lead screw positioning mechanism is U.S. Pat. No. 4,613,245 for DEVICE FOR CONTROLLING THE CARRIAGE RETURN OF A LEAD SCREW DRIVEN PRINTING HEAD (the '245 patent).
Prior lead screw print head drive mechanisms can introduce positional errors due to component imperfections and system inaccuracies. These imperfections and inaccuracies may include irregularities in drive system components, thread imperfections, axial misalignments and similar component and manufacturing-related variations. In a lead screw mechanism, these sources of positional error tend to be manifested in cyclical repetitions that correspond to the characteristics and gear ratios of the drive system componentry. In printing architectures that generate images using scan lines, these positional errors can introduce undesirable white space between adjacent scan lines and produce other printing artifacts that reduce image quality.
These positional errors can be controlled to some degree by the use of precision components and control systems in the drive mechanism and associated positioning assemblies. However, such precision components and control systems are more expensive and often more time-intensive to manufacture and assemble.
Accordingly, what is needed is a low cost, low complexity lead screw drive mechanism and positioning assembly for a print head that provides improved positional accuracy and overcomes the drawbacks of prior systems.